Alex, I guess once the channel is full, the water velocity will have reached an optimum speed. Engineering works that remove bends that have served to physically slow the flows, ...dredging, that increases the river profile are all designed to increase hydraulic capacity to discharge water to the sea as quick as possible at a higher velocity.
Taken to its extremes, the channel bed is concreted to reduce friction and maintenance making it an increasingly hostile environment for fish.
Any work undertaken in isolation usually creates knock-on problems up or downstream as does floodplain urbanisation.
In answer to your question, ..in theory if a small river and a larger deeper river both had the same unimpeded gradient fall, the larger river would flow the fastest due to a smaller percentage contact area to bank and bed resulting in less friction. When the river goes over fields the contact/ friction area would generally increase, given the depth to width ratio, and flows would be slower except for the original channel which will remain constant at bank high levels.
There is a good example of differing flows on the Broadmeads at Old Woking.
The natural loop of river with all of it's meanders and sharp bends looks positively tranquil in flood conditions compared with the man made straight as a rule flood alleviation channel ( Broadmead Cut ) that takes the shortcut across the loop. During the wet winter of 2000 (or thereabouts) the 150 odd acres of floodplain between the cut and river became a perpetually flooded lake , with hardly any discernible flow and a magnet for water fowl, whilst both river and especially the cut raged on.
Believe it or not, there are investment plots on the Broadmeads! They're going to need stilts I reckon.
